295/50R21 vs 285/55R21
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/50R21 | 285/55R21 | Change |
|---|---|---|---|
| Overall height | 32.6" | 33.3" | +2.2 % |
| Sidewall | 5.81" | 6.17" | +6.3 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 102.5" | 104.7" | +2.2 % |
| Revs per mile | 618 | 605 | -2.2 % |
| Axle height | 16.31" | 16.67" | +9.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 295/50R21. Fit 285/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,223 on the road, so it under-counts by 223. At a true 70 mph the first turns 721 times a minute, the second 706.